domingo, 10 de diciembre de 2017

How scientists study tropical climatic conditions from 5 million years ago?

One of the most important geologic and climatic events on Earth´s
history was the uplift of the Andes mountains, in the South American continent.
Nowadays, the tropical Andes are considered a hotspot of biodiversity for both
macro (vegetation) and microorganisms. They are also a key source of freshwater
to many South American countries. Once the Andes reached its present-day
elevation over 4000 m above sea level, 5 million years ago, a variety of newly formed
ecospaces developed. Among them, aquatic ecosystems such as lakes, streams and
marshes, were the most characteristic. However, very little is known about how these
ecosystems have evolved and their responses to past climate changes that could
serve as modern analogues for the ongoing climate change impacts.

When the Andes uplifted, climate was warmer due to the
higher concentrations of atmospheric CO2 (450 ppm vs 400 ppm) and
sea level was higher due to absence of ice caps in the Artic continent.
Therefore, the implications of understanding past tropical climate behavior to
predict future climate change impacts on Andean freshwater ecosystems are clear,
partly because all such past climatic conditions are predicted to occur in the
most extreme scenarios drawn by the Intergovernmental
Panel of Climate Change reports (IPCC).

Using diatoms (unicellular microalgae) and ostracods (very
small invertebrates), a multidisciplinary team of geologists, climatologists
and biologists performed a study in the Andean Altiplano of Perú to better
understand how climate operates at the South American tropics. Both diatoms and
ostracods occupy a wide range of environmental conditions and have hard
extracellular walls, made of silica (diatoms) or calcium carbonate (ostracods),
which are preserved in the sediments when the organism die. By looking at these
structures we can identify the species. So, if their remains are recovered,
environmental and climatic conditions might be then inferred from the occurring
species. In other words, diatoms and ostracods are indicators of both modern
and past environmental conditions. The authors of this study collected sediment
and biological samples at the Descanso formation, a geological outcrop - defined as an
ancient rock exposed on the surface of the Earth - that was deposited between 5
and 3.9 million years ago, during the so called Pliocene geologic epoch.

The diatom Pseudostaurosira
zeilleri, indicative of high saline-alkaline shallow waters

The results of this study, recently published in Palaeoegeography, Palaeoclimatology, Palaeoecology
journal, reveal that fluvial conditions prevailed just after the Andes uplift.
Then, lakes and marshes were established as result of increased precipitation
and direct underground water input due to tectonic activity. Basically, diatoms
and ostracods record indicated that lakes were developed under a higher
precipitation and seasonality regime compared to modern conditions. In this
region of the central Andean Altiplano, Pliocene climatic conditions were
characterized by enhanced precipitation during the rainy season, intensified
under the El Niño-like conditions
that are hypothesized to have prevailed at that time, between 5 and 3.9 million
years ago, driven by a warmer atmosphere (~3ºC).

Example of a surveyed lake in the Peruvian Altiplano. Photo: Maria I Velez.

Recent climatic models indicate that high altitude tropical
regions will be much more affected by a warmer atmosphere than low altitude
temperate regions. If model predictions are correct, these changes will have
important consequences for high altitude Andean freshwaters such as streams,
lakes and even glaciers. As these freshwater ecosystems feed many reservoirs, more
than 40 million people from Colombia, Ecuador, Peru and Venezuela that directly
depend on these water resources, will be seriously affected. The present study
contributes to a better understanding of the aquatic ecosystem response to climate
change in highly sensitive ecologically regions like the tropical Andes, and highlight
the importance of fossil records in addressing modern and future socio-environmental
research questions.

You can read the summary in Spanish and Portuguese (thanks to Isabel Fernandes for the PT translation)